Accurate qLBL acoustic positioning of multiple, fast moving underwater targets in confined waters
Q Quick-LBL (qLBL) is an underwater acoustic positioning system designed to track a target in confined waters (test or industrial pools, port areas) with high update rates. Same as a conventional LBL (long-baseline) system, the qLBL system comprises a number of baseline nodes, deployed around the perimeter of confined water area. In the experiments were used six nodes for the first system developed, four baseline nodes is the minimum required for system operation. Positions of the baseline nodes in the local coordinate frame were measured and then more accurately estimated during the system calibration. The system times of all baseline nodes were in precise sync with each other (due to timekeeping with integrated atomic clocks). The system allowed for high update rates of target’s positions, as all acoustic communication was unidirectional: the modem on the target acted as a pinger and transmitted short acoustic pulses. The S2C communication technology allowed the baseline nodes to detect these pulses from the target even in high-multipath environments. The baseline nodes estimated ranges to the target based on time differences of the target signal arrivals (multilateration) – the time differences were known since the internal clocks of the baseline nodes and the mobile target were synchronized with 1 PPS signals. The position of the target was calculated along with an estimation of positioning error. In different experiments in confined waters (pools) the system allowed for update rates of target position of up to 10 Hz, achieved for 1 target in a narrow, 100 m long pool. The update rate depended on the number of targets and the maximum distance towards the target in the baseline area. Multiple targets were supported, the update rate resource is shared between the targets – i.e. when one target can be tracked at 10 Hz, two targets would be tracked at 5 Hz.